The present invention relates to a white balance correction apparatus in electronic cameras producing color video images with an excellent color balance even when an outdoor main subject in daylight is taken from the inside of a house under artificial light through a window.
Generally, electronic cameras such as electronic still cameras, video cameras, and so forth incorporate therein an automatic white balance correction apparatus which automatically controls gains of signals of primary color components of light, provided by an image sensor such as CCDs, according to color temperatures of light entering the image sensor from a scene or an environment including the scene, thereby producing an image with naturally balanced color.
Such an automatic white balance correction apparatus is very useful and effective, especially when a subject is photographed by the electric camera under artificial light, such as emitted from a fluorescent lamp. Though an image taken under light of a fluorescent lamp is generally tinged with blue because the color temperature of light from the fluorescent lamp is higher than that of standard daylight, the automatic white balance correction apparatus can correct undesirable color balance of the image.
When a subject is illuminated with a fluorescent lamp, the amount of light from the subject illuminated with the fluorescent lamp is not always sufficient to photograph an image of the subject by the electronic camera and also, the light flickers at a commercial power-frequency (50 or 60 Hz). Therefore, because the color temperature of the light from a scene under lighting of fluorescent lamps is hard to measure precisely in the same way as available for the scene in daylight, the automatic white balance correction apparatus is used to judge automatically whether the light source illuminating a scene is a fluorescent lamp or not by detecting the flickering of light. When the white balance correction apparatus judges the light source to be a fluorescent lamp, it operates in a fluorescent mode to measure the color temperature without the influences of the flickering of light.
There are two types of methods of measurement of color temperatures; one measurement makes use of light which comes from a scene to be taken through a taking lens system of the electronic camera, the other measurement makes use of light from an environment including the scene.
The former method of measurement has an advantage that even if there is an artificial light source in the environment but out of a scene to be taken, the light source does not affect this measurement. However, if the color balance of the scene is improper, desirable color correction is difficult to achieve due to color failure. On the other hand, the latter has an advantage that, even if there is an artificial light source, the color correction is not affected by this color failure. However, if a subject out of doors in daylight is photographed with the electronic camera through a window, a video image of the scene is imbalanced in color because the automatic white balance apparatus is set in the fluorescent lamp mode of operation.